This invention relates to electrochromic display apparatuses and particularly concerns electrochromic display apparatus employing an electrochemical reaction.
The electrochromic display apparatuses are expected to have a bright future since the color of indication is brilliant, necessary voltage and current are small and there is no restriction in the observation angle. However hitherto, the life of the electrochemical display apparatus have not been satisfactorily long.
The electrochromic display apparatuses are divided into two groups, namely, physical type electrochromic display apparatuses and electrochemical type electrochromic display apparatuses. Each of the groups are further divided into an inorganic type and an organic type. Electrochromic substances used in the abovementioned various types of apparatus are shown in the Table 1 below.
TABLE 1 ______________________________________ types electrochromic substance ______________________________________ Physical type inorganic WO.sub.3, MoO.sub.3, TiO.sub.2 electrochromic organic metanitroanilins, merocyanine dyes apparatuses such as indophenol blue. Electro- inorganic ions of metal (Ag.sup.+, Na.sup.+), ions of chemical type non-metal (I.sup.-), polytungsten- electro- anions. chromic organic viologen compounds such as hexyl apparatuses viologens, heptyl viologens, octyl viologens, benzyl viologens; redox dyes such as leucodyes and tetraalkyl-p-arylenediamines; PH indicators such as phenol- phthalene, phenol red, cresol red. ______________________________________
Among the abovementioned listed electrochromic display apparatuses, the apparatuses of the electrochemical type make use of the change of light absorption or light reflection of the substance formed on an indication electrode by the electrochemical reaction of oxidation or reduction of the electrochromic substance.
A typical example of the prior art devices of the electrochromic display apparatus of the elctrochemical type is elucidated referring to
FIG. 1, which is a circuit diagram of the apparatus of the prior art, and
FIG. 2 which shows a voltage wave form of pulses to be impressed across the electrodes 1 and 2.
As shown in FIG. 1, in a container 4, for example, of glass, which contains electrochromic substance 3 or solution in the liquid phase, gel phase or solid phase, a pair of chemically stable electrode, namely, an indication electrode 1 and a counter-electrode 2 are disposed with a specified gap inbetween. Therefore, the gap between the electrodes 1 and 2 is filled with the electrochromic solution 3. The electrodes 1 and 2 are connected to a D.C. power source 5 through a polarity inversing switch 6. By impressing a D.C. voltage 7 shown in FIG. 2 of a polarity selected by the switch 6, for a specified time periode on the electrodes 1 and 2, cations A.sup.+ and anions B.sup.-, respectively, which have been produced by a dissociation shown by the belowmentioned reaction formula (1) of a electrochromic substance AB, there results a reduction reaction of the belowmentioned reaction formula (2) at the cathode and an oxidation reaction of the belowmentioned reaction formula (3) at the anode, respectively. EQU AB.fwdarw.A.sup.+ +B.sup.- ( 1) EQU A.sup.+ +e.sup.- .revreaction.A (2) EQU B.sup.- -e.sup.- .revreaction.B (3).
Accordingly, reduction product A onto the cathode and oxidation product B onto the anode, respectively are formed.
The polarity of the voltage impressed on the electrodes are selected in a manner that, when the reduction product A is used for the indication, then the indication electrode is connected as the cathode and, when the oxidation product B is used for the indication, then the indication electrode is connected as the anode. By opening the polarity inversing switch 6 after the preset time, the redox reaction stops and the reduction product A and the oxidation product B are retained on the abovementioned respective electrodes. Then, by inversely connecting the inversing switch 6 after a preset time period, an inverse polarity voltage 8 shown in FIG. 2 is impressed on the electrode, and the reverse reaction takes place, thereby forming A.sup.+ cations from the reduction product A and B.sup.- anions from the oxidation product B and dissolving them from the electrodes, and resultantly, the indication is erased.
In such a conventional electrochemical electrochromic display apparatus, as described in the above, the pulses impressed on the electrodes 1 and 2 have the positive writing pulse 7 and the negative erasing pulse 8. Since the voltage of each electrode is inversed at every writing-in and erasing-off, the reduction product A and the oxidation product B are alternately produced and dissolved on the same indication electrode 1. Since some small amounts of the reaction products are remaing on the indication electrode, both remaining products may coexist on the electrode. Such a reduction product A and the oxidation product B are generally liable to chemical reaction with each other as shown by the belowmentioned irreversible reaction formula (4), and therefore, such a series of the impressing of the alternate polarity of pulses is likely to form an undesirable irreversible reaction product C deposited on the indication electrode. EQU A+B.fwdarw.C (4).
Such a reaction product C coats and contaminates the surface of the indication electrode thereby causing an increase in the resistance of the apparatus. Therefore, the reactions of reduction and oxidation at the electrode surface are hindered thereby adversely effecting normal electrochemical electrochromic operation of the apparatus.